Look at other people's skins that you like. Notice how they are shaded and take bits and pieces to incorporate into your own work. Also, SPEND TIME ON YOUR WORK. Such as skins... it's ok if it takes a few hours or even days to make one... make a base of colors, then focus on each part at a time, such as an arm or a leg, separately and work to shade each of them with details. If you are unsure how to add details look up what others did. Just don't copy(you can copy to practice if you don't use or post them though).